It is highly desirable for pneumatic tires to have good wet skid resistance, low rolling resistance and good wear characteristics. It has traditionally been very difficult to improve the wear characteristics of a tire without sacrificing its wet skid resistance and traction characteristics. These properties depend, to a great extent, on the dynamic viscoelastic properties of the rubbers utilized in making the tire.
In order to reduce the rolling resistance and to improve the treadwear characteristics of tires, rubbers having a high rebound have traditionally been utilized in making tire tread rubber compounds. On the other hand, in order to increase the wet skid resistance of a tire, rubbers which undergo a large energy loss have generally been utilized in the tire's tread. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubbery material for automobile tire treads.
It is conventionally believed to be desirable for styrene-butadiene rubber which is utilized in tire tread compounds to have a high level of vinyl content (1,2-microstructure). It is also generally desirable for the repeat units which are derived from styrene to be randomly distributed throughout the polymer chains of the rubber. To achieve these objectives, styrene-butadiene rubbers are often synthesized by solution polymerization which is conducted in the presence of one or more modifying agents. Such modifying agents are well known in the art and are generally ethers, tertiary amines, chelating ethers or chelating amines. Tetrahydrofuran, tetramethylethylene diamine (TMEDA) and diethyl ether are some representative examples of modifying agents which are commonly utilized.
U.S. Pat. No. 5,284,927 discloses a process for preparing a rubbery terpolymer of styrene, isoprene and butadiene having multiple glass transition temperatures and having an excellent combination of properties for use in making tire treads which comprises terpolymerizing styrene, isoprene and 1,3-butadiene in an organic solvent at a temperature of no more than about 40° C. in the presence of (a) tripiperidino phosphine oxide, (b) an alkali metal alkoxide and (c an organolithium compound.
U.S. Pat. No. 5,534,592 discloses a process for preparing high vinyl polybutadiene rubber which comprises polymerizing 1,3-butadiene monomer with a lithium initiator at a temperature which is within the range of about 5° C. to about 100° C. in the presence of a sodium alkoxide and a polar modifier, wherein the molar ratio of the sodium alkoxide to the polar modifier is within the range of about 0.1:1 to about 10:1; and wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.01:1 to about 20:1.
U.S. Pat. No. 5,100,965 discloses a process for synthesizing a high trans polymer which comprises adding (a) at least one organolithium initiator, (b) an organoaluminum compound, (c) a barium alkoxide and (d) a lithium alkoxide to a polymerization medium which is comprised of an organic solvent and at least one conjugated diene monomer.
U.S. Pat. No. 5,100,965 further discloses that high trans polymers can be utilized to improve the characteristics of tire tread rubber compounds. By utilizing high trans polymers in tire tread rubber compounds, tires having improved wear characteristics, tear resistance and low temperature performance can be made. Such high trans polymers include, trans-1,4-polybutadiene, trans styrene-isoprene-butadiene terpolymers, isoprene-butadiene copolymers and trans-styrene-butadiene copolymers.
U.S. Pat. No. 6,103,842 discloses a process for synthesizing a random styrene-butadiene rubber having a high trans content by a process which comprises copolymerizing styrene and 1,3-butadiene under isothermal conditions in an organic solvent in the presence of a catalyst system which consists essentially of (a) an organolithium compound, (b) a barium alkoxide and (c) a lithium alkoxide.